Ships and boats are tied to a dock by a line. The ships and boats must be, securely, tied to the dock until it is time to be released from the dock. Then, the line can be released from the ship or from the dock and the ship and boat allowed to move away from the dock.
Normally, the line is attached to the ship or boat and runs to the dock. On the dock there is a releasable hook. The releasable hook is of such construction as to be able to withstand the pulling on the line by the boat. Due to the action of the water, the boat is moved with respect to the dock and the line running from the boat or ship to the dock must be sufficiently strong so as not to break with this movement of the boat or ship and also the releasable hook must be of such construction as to not break or release the line upon the pulling of the line by the ship or boat.
The releasable hook on the dock has two main functions. One of these functions is to firmly position the line running from the ship and boat so that the line will not move away from the releasable hook and therefore the ship or boat will not move away from the releasable hook and the dock. A second function of the releasable hook is to, positively, release the line when the releasable hook is tripped so that the ship or boat can move away from the dock.